Albumin is a protein naturally found in the blood plasma of mammals where it is the most abundant protein. It has important roles in maintaining the desired osmotic pressure of the blood and also in transport of various substances in the blood stream. Albumins have been characterized from many species including human beings, pig, mouse, rat, rabbit and goat and it has been found that albumins from different sources share a high degree of structural relationship.
Albumin binds in vivo to the neonatal Fc receptor (FcRn) and this interaction is known to be important for the plasma half-life of albumin (Chaudhury et al 2003; Montoyo et al., 2009). FcRn is a membrane bound protein, and has been found to salvage albumin as well as IgG from intracellular degradation (Roopenian D. C. and Akilesh, S. (2007), Nat. Rev. Immunol 7, 715-725.). Thus, FcRn is a bifunctional molecule that contributes to the maintaining the high level of IgG and albumin in serum of mammals such as humans.
While the FcRn-IgG interaction has been characterized in the prior art, the FcRn-albumin is less well characterized. Data indicated that IgG and albumin bind noncooperatively to distinct sites on FcRn (Andersen et al. (2006), Eur. J. Immunol 36, 3044-3051; Chaudhury et al. (2006), Biochemistry 45, 4983-4990). It is known that mouse FcRn binds IgG from mice and humans whereas human FcRn appears to be more discriminating (Ober et al. (2001) Int Immunol 13, 1551-1559) and does not bind mouse IfG (Ober et al. (2001) Int Immunol 13, 1551-1559). Furthermore, human FcRn binds albumin from both mouse and human, whereas mouse FcRn does not bind human albumin (Andersen et al (2010) JBC).
Human serum albumin (HSA) has been well characterized as a polypeptide of 585 amino acids, the sequence of which can be found in Peters, T., Jr. (1996) All about Albumin: Biochemistry, Genetics and Medical, Applications, Academic Press, Inc., Orlando. It has a characteristic binding to its receptor FcRn, where it binds at pH 6.0 but not at pH 7.4. The serum half-life of HSA has been found to be approximately 19 days. A natural variant having lower plasma half-life has been identified (Biochim Biophys Acta. 1991, 1097:49-54) having the substitution D494N. This substitution generated an N-glycosylation site in this variant, which is not present in the wild type HSA. It is not known whether the glycosylation or the amino acid change is responsible for the change in plasma half-life.
Albumin has a long serum half-life and because of this property it has been used for drug delivery. Albumin has been conjugated to pharmaceutically beneficial compounds (WO0069902A), and it was found that conjugate had maintained the long plasma half-life of albumin so the resulting plasma half-life of the conjugate has generally been found to be considerably longer than the plasma half-life of the beneficial therapeutic compound alone.
Further, albumin has been fused to therapeutically beneficial peptides (WO 01/79271 A and WO 03/59934 A) with the typical result that the fusion has the activity of the therapeutically beneficial peptide and a long plasma half-life considerably longer than the plasma half-life of the therapeutically beneficial peptides alone.
Albumin has the ability to bind a number of ligands, and this property has been utilized to extend the plasma half-life of drugs having the ability to bind to albumin. This has been achieved by binding a pharmaceutical beneficial compound to a moiety having albumin binding properties. It is not clear what determines the plasma half-life of the formed conjugates or fusion polypeptides but it appears to be given by the albumin and the selected pharmaceutically beneficial compound/peptide they are composed of. It would be desirable to be able to control the plasma half-life of a given albumin conjugate or albumin fusion polypeptide so that a longer or shorter plasma half-life than given by the components of the conjugate/fusion can be achieved, in order to be able to design a particular drug or vaccine according to the particulars of the indication intended to be treated.